1. Field of the Invention
The present invention relates to a YUV-RGB digital conversion circuit which converts a digital luminance signal Y and digital color-difference signals U and V into digital color signals R, G, and B, an image display apparatus using the same, and an electronic apparatus using the image display apparatus.
2. Description of Related Art
As an electronic apparatus using an image display apparatus, for example, a projector will be given as an example.
A liquid-crystal display apparatus of this projector includes a liquid-crystal panel having a liquid crystal sealed between a pair of substrates, a signal processing circuit for performing signal processing, such as gamma correction or polarity inversion, suitable for driving the liquid-crystal panel, on an input RGB signal, and a driving circuit for driving the liquid-crystal panel on the basis of an output of this signal processing circuit.
Here, because of a demand for a liquid-crystal display apparatus with a smaller size, the signal processing circuit must be formed into an IC. Therefore, a digital RGB signal must be provided to the signal processing circuit of the liquid-crystal display apparatus.
The RGB signal provided to this liquid-crystal display apparatus is output from the control board of the main unit of the projector. This control board is provided with a YUV-RGB conversion circuit for converting a luminance signal Y and color-difference signals U and V into RGB signals. Here, in the control board, it is necessary to perform various processing on the RGB signal, and since a memory, such as a VRAM, is used for this processing, digital processing is suitable for the signal processing by the control board. If YUV-RGB conversion by the YUV-RGB conversion circuit is performed digitally, the efficiency is high.
The YUV signal and the RGB signal have the following relationship when each signal is assumed to be of 8 bits (=256 gradations):
xe2x80x83R=Y+(Vxe2x88x92128)xc3x971.371xe2x80x83xe2x80x83(1)
G=Yxe2x88x92(Vxe2x88x92128)xc3x970.337xe2x88x92(Uxe2x88x92128)xc3x970.698xe2x80x83xe2x80x83(2)
B=Y+(Uxe2x88x92128)xc3x971.733xe2x80x83xe2x80x83(3)
The value of 128, which is subtracted from the color-difference signals U or V, is the middle value of 256 gradations and differs depending upon the total number of gradations. The reason why the middle value of the total gradation value is subtracted from the color-difference signals U and V as described above is that each coefficient shown in equations (1) to (3) must be multiplied by a color-difference signal which becomes positive or negative, assuming to be zero when it has the middle value of the full gradation value.
Here, each of the coefficients multiplied by (Vxe2x88x92128) and (Uxe2x88x92128) includes a decimal, such as 1.371, 0.337, 0.698, or 1.733.
To realize a product of such decimals by logic, a method is known in which this decimal is expanded into the sum of 2xe2x88x92n (n is a natural number) and computed. For example, (Vxe2x88x92128)xc3x970.5=(Vxe2x88x92128)xc3x972xe2x88x921 can be determined by shifting the digital value of(Vxe2x88x92128) by one bit to the lower order. Similarly, (Vxe2x88x92128)xc3x972xe2x88x92n can be computed easily for each coefficient (xe2x88x92n) by shifting the digital value of (Vxe2x88x92128) by n bits to the lower order.
Each of the above-described coefficients is expanded to the sum of 2xe2x88x92n as described below.
1.371≈20+2xe2x88x922+2xe2x88x924+2xe2x88x925+2xe2x88x926+2xe2x88x927+2xe2x88x929+2xe2x88x9210+2xe2x88x9211+2xe2x88x9212+2xe2x88x9213+2xe2x88x9216+ . . . 
0.337≈2xe2x88x922+2xe2x88x924+2xe2x88x926+2xe2x88x927+2xe2x88x9210+2xe2x88x9214+2xe2x88x9216+2xe2x88x9217+2xe2x88x9219+2xe2x88x9224+2xe2x88x9225+ . . . 
0.698≈2xe2x88x921+2xe2x88x923+2xe2x88x924+2xe2x88x927+2xe2x88x929+2xe2x88x9211+2xe2x88x9212+2xe2x88x9219+2xe2x88x9225+2xe2x88x9226+2xe2x88x9230+ . . . 
1.733≈20+2xe2x88x921+2xe2x88x923+2xe2x88x924+2xe2x88x925+2xe2x88x927+2xe2x88x928+2xe2x88x929+2xe2x88x9211+2xe2x88x9214+2xe2x88x9216+2xe2x88x9217+ . . . 
Regarding the above-described coefficients, only approximated coefficients can be used as long as the number of expansion terms is finite. Here, if this coefficient is expanded to multiple terms, a more accurate value can be used, but the scale of the circuit becomes large. On the other hand, if the number of expansion terms is decreased too much in order to reduce the scale of the circuit, the computation error becomes larger. As described above, the number of expansion terms of the coefficient must be determined by taking both the scale of the circuit and the computation error into consideration.
Next, the scale of the computation circuit is considered after the number of expansion terms is determined. In the case where, for example, the coefficient 1.371 is expanded to seven terms and approximated in equation (1) described above, if each of these terms is added in sequence, six adders are required, and the scale of the circuit increases. Also, if, for example, the data is of 8 bits, the 20 term of the highest order requires 8 bits for only the integer part, and the 2xe2x88x928 term of the lowest order requires 8 bits for only the decimal part. During the computation process, 16 bits are required for the total of the integer part and the decimal part, and this causes the scale of the circuit to increase.
Accordingly, an object of the present invention is to provide a YUV-RGB digital conversion circuit capable of reducing the scale of a circuit by decreasing a number of adders for adding the terms such that a coefficient including a decimal to be multiplied by a digital color-difference signal is approximately expanded to a finite number of 2xe2x88x92n terms in each conversion section for converting a digital YUV signal to a digital RGB signal, and an image display apparatus and an electronic apparatus using the YUV-RGB digital conversion circuit.
Another object of the present invention is to provide a YUV-RGB digital conversion circuit capable of reducing the scale of a circuit by truncating unnecessary bits in a computation process in which each term of 2xe2x88x92n is added together, and an image display apparatus and an electronic apparatus using the same.
Still another object of the present invention is to provide a YUV-RGB digital conversion circuit capable of outputting an RGB signal such that the display is not inverted even if there is an input value other than a theoretical specified value, and an image display apparatus and an electronic apparatus using the same.
The invention is characterized in that, a YUV-RGB digital conversion circuit for converting a digital luminance signal Y and digital color-difference signals U and V into digital color signals R, G, and B includes a YV-R conversion section for converting a digital luminance signal Y and a digital color-difference signal V into a color signal R,
a YUV-G conversion section for converting a digital luminance signal Y and digital color-difference signals U and V into a color signal G, and
a YU-B conversion section for converting a digital luminance signal Y and a digital color-difference signal U into a color signal B,
each conversion section includes a plurality of bit-shift circuits, provided in each stage, for outputting an input signalxc3x972xe2x88x92k (k is a natural number such that kxe2x89xa6n) by bit-shifting an input signal by one or a plurality of bit-shifting in order to add the terms such that a coefficient including a decimal multiplied by a digital color-difference signal is approximately expanded to a finite number of terms of 2xe2x88x92n (n is a natural number); and
a plurality of adders, provided in each stage, for performing addition of the terms of two sets of an input signalxc3x972xe2x88x92k, whose value of the multiplier k is different, and
the addition of a combination such that the difference of each multiplier k of the two sets of terms to be added becomes the same is shared by one adder.
According to the invention, when, for example, a YV signal is converted into an R signal, for example, Vxc3x97(20+2xe2x88x922+2xe2x88x924+2xe2x88x925+2 xe2x88x926+2xe2x88x927+2xe2x88x928) is computed, and of this, for example, both 2xe2x88x927+2xe2x88x928 and 2xe2x88x925+2xe2x88x926 are additions of a first-power difference. Accordingly, initially, after Vxc3x972xe2x88x921 is obtained by using a bit-shift circuit of the first stage, the addition of Vxc3x972xe2x88x921 and Vxc3x9720 such that the difference of each multiplier k becomes a first-power difference is performed. If this V(20+2xe2x88x921) is shifted by the bit-shift circuit by five bits to the lower-order side, 2xe2x88x925+2xe2x88x926 is obtained. If it is shifted by another bit-shift circuit by seven bits to the lower-order side, 2xe2x88x927+2xe2x88x928 is obtained. As described above, since one adder can be shared for the addition of terms such that the power difference is equal, it is possible to reduce the scale of the circuit.
The invention is characterized in that, the plurality of adders are connected to multiple stages so that the addition of terms corresponding to a smaller term from among a plurality of terms of 2xe2x88x92n is performed with priority, and when the output of the adder of a previous stage is bit-shifted by a bit-shift circuit, a plurality of additions are performed while dropping the low-order bits such that there is no addend to be added in the addition or subsequent additions by the adder of the next stage.
According to the invention, since digits which are not related to the carry-over to the digit of the data of the final output can be truncated during computation, the number of computation bits is reduced, and the scale of the circuit can be reduced.
The invention is characterized in that, the YUV-G conversion section includes a plurality of adders for adding two sets of terms, the term of a color-difference signal Uxc3x972xe2x88x92i (i is a natural number such that ixe2x89xa6n) and the term of a color-difference signal Vxc3x972xe2x88x92j is a natural number such that jxe2x89xa6n), and the addition of a combination such that the difference (ixe2x88x92j) of each multiplier of two sets of terms is the same is shared by one adder.
In the YUV-RGB conversion section, U and V are used as the color-difference signals, and an adder for adding, for example, the first-power difference term of the digital color-difference signals U, and an adder for adding the first-power difference term of the color-difference signals V cannot be shared in this case because the input data are different from U and V. If it is constructed in accordance with the invention, since the color-difference signal Uxc3x972xe2x88x92i and the color-difference signal Vxc3x972xe2x88x92j can be input commonly to one adder, the number of adders is decreased, and the scale of the circuit is reduced.
The invention is characterized in that,
a carry-over signal, together with an addition output of predetermined bits, is output from the adder of the final stage, and
there is further provided a luminance-limit circuit for inputting an output of the adder of the final stage and for forcibly setting the addition output of predetermined bits to all 1 in accordance with the carry-over signal.
According to the invention, even if a value out of the specified range, exceeding a maximum value of the adder of the final stage, is output, the value can be forcibly corrected to a maximum value by the luminance-limit circuit, and the image quality can be improved.
The invention also provides a YUV-RGB digital conversion circuit characterized in that,
each conversion section includes a computation unit for subtracting a predetermined gradation value from a color-difference signal U or V, a negative-sign signal indicating that the output of the computation unit is negative, together with an addition output of predetermined bits and a carry-over signal, is output from the adder of the final stage, and the luminance-limit circuit forcibly sets the addition output of predetermined bits to all 0 in accordance with the negative-sign signal.
According to the invention, even if the output of the adder of the final stage becomes a negative value as a result of an input that is out of the specified range, since the output is forcibly corrected to a minimum luminance value by the luminance-limit circuit, the image quality can be improved.
The invention is characterized in that, the total number of expansion terms in which a coefficient to be multiplied by a digital color-difference signal is approximately expanded to a plurality of terms of 2xe2x88x92n is set to a finite number such that the SN ratio of each signal of RGB is 60 dB or more.
According to the invention, even if the number of expansion terms is finite, accuracy such that the SN ratio is 60 dB or more can be obtained, and an image having an image quality of a predetermined level or greater can be reproduced while YUV-RGB conversion is being performed digitally.
The invention also provides an image display apparatus and an electronic apparatus including a YUV-RGB digital conversion circuit in accordance with the invention.